FIGS. 1-5 provide simplified illustration of some events which occur in electric motors, and give some possible explanations of vibration and noise.
FIG. 1A illustrates permanent magnets 3, having poles north N and south S, as contained within a permanent magnet electric motor (motor is not shown). FIG. 1B illustrates an armature 6, which includes a single-turn coil 9 and a commutator 12. In operation, brushes 15 contact the commutator 12. FIG. 1C illustrates the components of FIGS. 1A and 1B when assembled.
FIG. 2A illustrates magnetic field lines 18 produced by the magnets 3 of FIG. 1A. FIG. 2B illustrates current 21 induced by voltage V+ applied to the brushes 15, and also the magnetic flux lines 24 which accompany the current 21. FIG. 2C is a cross-sectional view of FIGS. 2A and 2B, with some of the flux lines 24 removed, and with the brushes 15 shown in contact with the commutator 12.
FIGS. 3A through 3F show the components of FIG. 2C in assembled form, and show how the magnetic flux 24, produced by the armature 6, rotates as the armature 6 rotates. In FIG. 3A, the flux 24 is directed to the left, and does not cross the south pole S. (In actual practice, some leakage flux may cross the south pole, but FIG. 3A is a simplification, used to illustrate major principles.)
In FIG. 3B, the armature 6 has rotated clockwise, and the armature's flux 24 occupies the position shown. In FIG. 3C, the armature flux 24 penetrates the south pole S.
In FIG. 3D, the armature flux 24 has disappeared, because the commutator 12 is no longer in contact with the brushes 15. In FIG. 3E, the armature flux 24 has re-appeared, because the commutator re-contacts the brushes 15. However, the flux 24 has reversed in direction, as indicated by a comparison of FIG. 3E with FIG. 3C. FIG. 3F indicates the position of the armature flux 24 a time later than in FIG. 3E, wherein the flux does not penetrate the north pole N.
The sequence of FIG. 3 provides a simple explanation of one cause of vibration. The sequence of FIGS. 3B through 3F show the following events:
Figure Event 3B No penetration of south pole. 3C Penetration. 3D No penetration. 3E Penetration, but reversed in direction. 3F No penetration.
The sequence can be characterized as a repeated sequence of two events: flux penetration of the south pole S, followed by removal of penetration.
In effect, a magnetic field is repeatedly applied, and then removed, from the south pole S. The application of the magnetic field applies a force to the south pole S. The removal of the magnetic field removes the force. The sequence of
. . . force . . . no force . . . force . . . no force
is believed to cause vibration of the south pole S. Similar events occur with respect to the north pole N.
A second cause of vibration can be explained with reference to FIGS. 4 and 5. In FIG. 4A, an actual armature 6 comprises a rotor 30 containing slots 33, which hold conductive bars 36 (also called armature windings). Additional conductors, indicated by the dashed lines 39, form a conductive loop, analogous to loop 9 in FIG. 1B.
FIG. 4B shows the slotted rotor 30 in cross section, and includes the conductive bars 36. When current passes through the loop comprising bars 36 and dashed lines 39 in FIG. 4A, the flux lines 40 shown in FIG. 5A are generated. Two positions which the slotted rotor occupies during rotation are shown in FIGS. 5B and 5C.
A significant feature of these two positions is that the flux lines must traverse different numbers of slots en route to the south pole S. That is, different flux lines follow paths through different materials. Consequently, different flux lines apply different forces to the south pole S. These differences can also cause vibration, as will now be explained.
The slots 33 in FIG. 5A act as an air gap, and reduce the strength of the flux lines 40. (Even though the slots 33 contain the conductive bars 36, the slots can be viewed, for present purposes, as being filled with air, because the magnetic permeability of the conductive bars is close to that of air, when compared with the permeability of the material of which the rotor 30 is itself constructed.
How an air-gap can change a magnetic field can be explained by an analogy. When a hand-held magnet is brought two inches from a steel nail, the nail hardly "feels" the magnet, because of the large, two-inch, air gap. However, when the magnet is brought sufficiently close to the nail, the nail snaps into contact with the magnet. The very small air gap, created when the magnet approached the nail, caused the strength of the flux lines (more precisely, the magnetic flux density) to increase.
Similarly, when the rotor 30 is in the position shown in FIG. 5B, the flux lines must pass through three slots, or air gaps, indicated in insert I, en route to the south pole S. In contrast, in FIG. 5C, the number of slots increases from three to four, as indicated in insert I2.
In effect, the air gap between the armature and the south pole S has increased from FIG. 5B to FIG. 5C. Consequently, the "pull" which the rotor 30 applies to the south pole S, because of the flux lines 40, decreases in FIG. C, compared with FIG. 5B, because of the increased air gap, similar to the case of the steel nail.
Therefore, as the armature 30 rotates, the number of slots, through which the flux lines must travel en route to the south pole S, changes, thereby changing the magnetic force applied to the south pole S. This changing magnetic force induces vibration. Some components of the vibration lie within the range of human hearing, and are perceived as audible noise.
A similar analysis applies to the north pole N.